Procedure for controlling an elevator

ABSTRACT

A velocity-controlled elevator drive including an a.c. motor (28) driving elevator machinery regulated by means of a frequency converter (26) supplying the motor (28) with a controlled frequency and voltage. The load condition of the elevator is measured by means of a load weighing device (32) in the elevator car. A power limit (P A ) is input to the elevator machinery as a reference value and the speed reference (38) given to the frequency converter is determined on the basis of the power limit (P A ) and the load condition.

BACKGROUND OF THE INVENTION

The present invention relates to a procedure for regulating avelocity-controlled elevator drive in which an a.c. motor driving theelevator machinery is controlled by a frequency converter feeding themotor with a controlled frequency and voltage, said elevator drive beingprovided with devices detecting the load condition of the elevator.

An objective in the control of an elevator in a normal situation is todrive the elevator in such a way that, each time the elevator isoperated, it will run through the distance between the starting floorand the target floor as fast as possible. Therefore, the elevator motoris generally so controlled that the acceleration, deceleration and speedof the elevator are in all circumstances as high as the machinerypermits without causing inconvenience to passengers. For the control, itis required that the electric network supplying the elevator driveshould produce sufficient power in all situations during elevatoroperation. In normal use, this is generally no problem.

When a disturbance occurs in the supply of power, the elevator will notwork in the intended manner. To cope with power failures, elevators areprovided with safety equipment enabling the elevator cars to be drivento landings. A longer break in the supply of electricity requires theconnection of a reserve power system, which is generally designed tokeep about one in four elevators available for use by passengers. Inthis case, the transport capacity of the elevators is dramaticallyreduced.

Disturbances may appear in the supply of electric energy even if noactual power failure occurs. The voltage in the electric supply networkmay fall below the nominal value or the frequency variations may exceedthe allowed limits. In such cases, the protective devices used in theelectric network and by the consumers of electricity are generallyactivated when certain preset limit values are reached. In elevatordrives, such situations may occur in areas where the electricitydistribution network is weak and also during construction when power issupplied by a temporary electricity supply system insufficient incapacity. When the voltage in the network falls, the load capacity ofthe network is generally reduced, so that a load of normal magnitudewill cause an overload on the network, resulting in a further fall inthe voltage, activation of protective equipment and break-off of power.

From patent specification U.S. Pat. No. 5,229,558, Kone Elevator GmbH, asolution is known in which the elevator is driven at a lower speedand/or acceleration when the supply voltage falls, correspondinglyreducing the power requirement. However, this specification does nottake the real power need of the elevator into account, but the transportcapacity, i.e. the travelling speed of the elevator is reduced on-thebasis of the condition of the electric network.

SUMMARY OF THE INVENTION

The object of the present invention is to achieve a newvelocity-controlled elevator drive which works optimally when thenetwork has a limited power supply capacity, e.g. during the use of areserve power supply. A further object is to achieve a procedure forcontrolling the elevator motor that does not impose on the network aload exceeding the network tolerance but allows a maximal driving speedin different load situations. The procedure of the invention ischaracterized in that a power limit is input to the elevator machineryas a reference value and that the speed reference given to the frequencyconverter is determined on the basis of the power limit and the loadcondition.

According to an embodiment of the invention, the power limit is given asa relative value in relation to the nominal power of the elevator.According to another embodiment of the invention, the load condition isdetermined from the measurement signal of the load weighing device ofthe elevator. In a third embodiment of the invention, the power limit isdetermined according to the power supply capacity of the network.

With the invention, all energy available to the elevator drive isoptimally utilized. This has a special importance in a reserve powersituation, where the power available is limited to a clearly lower valuethan normal.

In the solution of the invention, the motor drive in the elevatorcontrol system is able to decide its running speed by itself inaccordance with conditions given. An advantageous condition mode is touse relative power. By virtue of the properties of a new type offrequency converter used, the elevator can be started with 12-25% ofnominal power even under the heaviest load conditions. However, this hasthe result that an empty elevator moves very slowly in the downdirection. If there are passengers in the elevator car, the powerrequired to drive downwards is reduced because the elevator is balancedto about 50% by the counterweight. In rescue operation, when the load isclearly over one half of the nominal load, mainly depending on theefficiency of the machinery, the elevator no longer needs power to movethe car. However, motor magnetization and the control equipment require10-25% of the nominal power.

For example, in the case of an elevator group comprising four elevatorsin which the reserve power capacity is designed on the principle typicalof mid-sized buildings, the available energy is sufficient for oneelevator in all operating situations. By using the solution of theinvention, each one of the elevators can be allotted 25% of the nominalpower. Depending on the load conditions, some or even all of theelevators can drive at full speed.

A significant advantage provided by the invention when applied inconnection with reserve power operation is a feeling of safety createdin the passengers, which is achieved by the fact that the elevatorsstart moving again immediately after a power failure after the lightshave been turned on again. Alternatively, depending on market needs andthe resources available, part of the advantage regarding quality ofservice can be translated into a saving in expenditure and the presentlevel of service can be attained for a considerably lower price. Thisadvantage can be achieved e.g. in high-rise residential buildings, whichgenerally have two elevators, which means that the waiting time is noproblem, and the advantage is created by the fact that the nominal powerof the reserve power system can be lowered either to about one halfwithout significantly reducing the level of service quality or to aboutone fourth of the present power level while still guaranteeing rescueoperation, though slow, with all elevators in all situations.

The invention provides a particularly great advantage in areas wherepower failures are very common. In this case, the solution of theinvention allows almost normal or quasi normal elevator operation.Therefore, the abnormal situation does not necessarily require specialinstructions to be given, nor does it affect the behavior of passengers.As compared with a purely battery operated solution, the inventionallows savings to be made in the costs of establishment and maintenanceof an energy storage. Further advantages are achieved in the supply ofelectricity to the control and peripheral apparatus.

Another field of application where the invention offers a particularlygreat benefit are fire situations in very high buildings. In suchbuildings, so-called gearless elevators are used which have such a highcoefficient of efficiency that, even with current technology, it makessense to supply energy back into the network e.g. when the elevator isdriving downwards with full load or upwards with an empty car. At best,the electric power returned corresponds to 90% of the nominal power. Forthis purpose, the motor drives are provided with a so-called controlledmains bridge, which produces a current of correct frequency, form andvoltage.

In a fire situation, the elevators can utilize the energy produced byother elevators via the internal network of the building, and thus allelevators can in this case drive practically at full speed all the timebecause in a rescue situation the cars generally travel down with fullload and up with an almost empty car, one fireman being generally alwayspresent in the car in such situations. The power generated by the otherelevators prevents the occurrence of overload on the reserve powersystem if the elevator machinery is temporarily put on heavy duty e.g.when the elevator is driving down with an empty car.

A further advantage in a fire situation is that, if the elevators can berun at full capacity during rescue work, they can even generate asignificant amount of extra power for other equipment in the building,such as normal lighting and pumps. Therefore, by using the solution ofthe invention, it will be beneficial to change the basic assumptions inthe planning of rescue work and demand that full elevator service beavailable in high-rise buildings in the event of a fire and when rescuework relies on reserve power. This can be realized without significantlyincreasing the total costs.

The power limit can be set proportionally among the elevators inoperation. In areas suffering from insufficient supply of electricpower, this allows the power limit to be determined by considering otherprimary loads on the network or, if the power available varies with thetimes of the day, the power limit can also be adjusted according to thediurnal rhythm.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described by referring to FIG. 1,which presents an elevator drive according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The hoisting motor 28 moves the elevator car 6 and counter weight 8 bymeans of elevator hoisting ropes 4 and a traction sheave 2 coupled tothe motor shaft either directly or via a gear system, in a manner knownin itself in elevator technology. The frequency converter 26 isconnected to the power supply via three-phase conductors 40 and to themotor 28 via three-phase conductors 41. The elevator control system forits part takes care of the movements of the car/cars in accordance withthe calls given by passengers and the internal instructions within theelevator system. The implementations of these vary considerablydepending on the application and do not affect the action of the presentinvention. Each elevator has an individual nominal power, although theelevator group may of course consist of identical elevators of standarddesign.

The elevator load is measured by means of a load weighing device 32mounted in the elevator car 6. Using the weight data, unit 37 generatesa load signal 36 based on the masses of the mechanics and components ofthe hoisting system of the elevator. The load data indicates the loadtorque acting on the shaft of the hoisting motor, i.e. the loadcondition. The load torque depends on the masses of the counterweight,car and ropes as well as the suspension ratios of the ropes and thetransmission ratios of the gear system.

In a normal frequency converter controlled elevator drive, the motor isfed with a voltage of controlled frequency, which develops a sufficienttorque for the desired acceleration and travelling speed. In a driveunder four-quadrant control, when the motor is working in generatormode, the power generated by the motor can be returned into the supplynetwork. Alternatively, the energy generated, or part of it, isconverted into heat in resistors. The frequency converter is suppliedwith input data representing the actual values of the travelling speedof the elevator or the rotational speed of the motor, the load or torqueand the voltage and possibly the current.

In a solution utilizing the invention, the frequency converter consistsof a mains bridge 42 connected to the supply network and a motor bridge46 connected to the motor. The motor bridge and the mains bridge areconnected by a d.c. intermediate circuit, with a capacitor 44 connectedbetween the intermediate circuit conductors 43 and 45. The two bridgesare composed of controlled switches implemented e.g. as IGBTs. Thebridges are controlled by a speed regulator 48, and the control is soimplemented that the power supplied to the motor and the supplyfrequency as well as the power returned into the network are inaccordance with the requirements of the operational situation. Theenergy stored in the intermediate circuit capacitor is utilized to copewith rapid load changes.

In each operational situation, the elevator is assigned a maximum powerP_(A) and a reference value for the rotational speed is determinedaccordingly. The allowed output power value, which is obtained from apower limiter 33, is e.g. one quarter of the nominal power of theelevator when the elevators are operated by the power generated by areserve power generator. The allowed maximum output power value can alsobe defined by other means, such as a parameter given to the elevatorcontrol system.

The size of the counterweight used in the elevator drive is so chosenthat, when the car load amounts to half the nominal load, a state ofequilibrium prevails on the shafts of the traction sheave and theelevator motor. When the car load is smaller, a torque acting in thedirection of the counterweight is present on the motor shaft, and whenthe car load exceeds half the nominal load, a torque acting in thedirection of the car is present on the motor shaft. Thus, the loadweight data provides a quantity directly proportional to the torque, andthe driving power required by the machinery is proportional to thevelocity and torque, or P=wT_(z) and further w=P/T. By expressing thesequantities as relative values com30 pared to the nominal values, weobtain w_(r) =P_(r) /T_(r), where the subscript r means a relative valueor P_(r) =P/P_(N). Limiting the power to 25% of the nominal powertherefore means a relative power value P_(r) =0.25. Thus, the referencefor rotational speed is obtained directly from the power limit and theload weight data. The allowed power limit is given as a relative valuecorresponding to the proportion of reserve power, i.e. to the ratio ofthe reserve power allotted to the elevator to the nominal power. Whenseveral elevators are connected to the same reserve power generator,each elevator can be assigned an individual power limit, which meansthat the elevators share the total reserve power designed for theelevator drive.

The signal determining the allowed power P_(A) and the load data(conductor 36) from the unit 37 are input to a divider 34, whichdetermines the reference speed ω_(ref) =P_(A) /T_(L) possible with theavailable power, where T_(L) is the load data. The speed referenceω_(ref) determined by the divider 34 is taken via conductor 38 to aspeed regulator 48 in the frequency converter 26, whose output the speedregulator adjusts accordingly. Thus, the power taken by the frequencyconverter form the network remains within the prescribed limits. Atachometer 31 connected to the motor shaft provides the actual speedvalue (ω_(act), which is taken via conductor 39 to the speed regulator48.

Depending on the load and travelling direction of the elevator,different load conditions can be distinguished. When the elevator istravelling with the car about half full, the load torque is very low andthe power limit set according to the previous paragraph practically doesnot reduce the travelling speed at all. When the elevator is drivingdownwards with an empty car or upwards with a full car, the load is at amaximum and the travelling speed is reduced according to the powerlimit. The most advantageous situation in respect of energy consumptionprevails when the elevator is travelling in the up direction with anempty car or in the down direction with a full car. In this situation,the power limit does not actually impose a limit on the driving speed ofan individual elevator, but the elevator motor is working in generatormode, generating power that has to be either consumed or returned intothe network. Of course, in all situations the motor has to produce thepower for its magnetization and power dissipation.

When the elevator motor is operated in generator mode, it isadvantageous to return the power generated into the network, so theenergy can be used by other equipment connected to the reserve powernetwork. If this is not possible, the power is dissipated in resistors.Another possibility is to operate the elevator in place, in which casethe motor is fed with a zero-frequency current corresponding to thestarting torque.

A relative power limit can be determined in several ways within theframework of the invention. Besides a preset relative value, the powerlimit may also be a function of a quantity representing the condition ofthe network. When the network voltage falls, this causes a stepwisereduction of the power limit.

Although the power control as presented in FIG. 1 is based on separateregulation of the elevators, it makes it possible, by monitoring thepower consumption of different elevators, e.g. those belonging to thesame elevator group, to alter the power limit for each elevatoraccording to the load condition. The torque required for start-up has tobe generated to enable the elevator to start moving. The speed andtransport capacity of the elevator, i.e. the number or rather mass ofpassengers times the floor distance travelled per unit of time, isdetermined individually for each elevator. As the power limit isindicated as an amount of power consumed by the elevator, it does notlimit the speed when the motor is working in generator mode. An elevatortravelling with a full load in the down direction, which is the usualsituation during evacuation, is advantageous in respect of powerconsumption as stated before and in fact generates power as the motor isworking in generator mode. The motor can be run at full speed, whichmeans that the transport capacity is at a maximum, i.e. the elevator istravelling with maximum load at full speed. The power thus generatedmust be consumed in some way or returned into the network. On the otherhand, when the load is small, only a low speed is allowed in the downdirection. In contrast, an empty car in the up direction or, as is oftenthe case in an emergency, a car with one rescue worker in the updirection provides a similar advantage, as stated above.

In the above, the invention has been described by the aid of some of itsembodiments. However, the examples are not to be regarded as limitingthe sphere of patent protection, but the embodiments of the inventioncan be varied within the limits defined by the following claims.

We claim:
 1. A velocity controlled elevator drive comprising:an elevatormotor driving an elevator; a detector detecting a load condition of theelevator; a power limiter determining a power limit representing a shareof reserve power allocated to said elevator; and a frequency convertersupplying the elevator motor with a controlled frequency and voltagebased on a reference speed as determined from the power limit defined bysaid power limiter and the load condition detected by said detector. 2.Elevator drive according to claim 1, wherein the power limit is arelative value in relation to nominal power of the elevator.
 3. Elevatordrive according to claim 1, wherein the load condition is determinedfrom a measurement signal give by a load weighing device in an elevatorcar.
 4. Elevator drive according to claim 1, wherein the power limit isdetermined according to power supply capacity of a network.